In the determination of pectin content in plant, it is generally performed by chromogenic method. However, during the chromogenic determination, pigments will probably interfere the chromogenic determination result due to its optical absorbance, and in the meanwhile, sugars would react with chromogenic reagent to generate color substances, which would also cause interference to chromogenic determination. Therefore, during the determination process, pre-treatment should be performed including sugar removal, pigment removal, etc. . . . For determining pectin content in plant sample, conventional pre-treatment is as follows: heating the powdery plant sample with 80% ethanol solution to reflux for 1 hour followed by filtration to obtain residue for further extraction. This method is generally called as ethanol treatment method, However, the deficiency of such ethanol treatment method is that such method could only remove the micromolecular water-soluble sugars, and has certain limitation in pigment extraction. Furthermore, when treating the residue, acid solution is sometimes required, which makes the extracted polysaccharides hydrolyzed and thus extracted. In the meanwhile, cell walls are broken due to the effect of acid, which makes the previously dissolved pigment be extracted and thus influences the final results of determination. After pre-treating process, the resulting sample is determined. Existing determination method is gravimetric method. Since pectin is not polymerized from pure galacturonic acid, when determining by gravimetric method, neutral sugars and alpha-L-(1→2) rhamnoses on the branched chains, as well as methylated groups and acetylated groups, are simultaneously precipitated, the weight thereof is inevitably more than the cumulated amount of galacturonic acid. Therefore, the determination results of gravimetric method could not imply the galacturonic acid content in sample, that is, it could not imply the content of galacturonic acid branched chain skeleton used for the characterization of pectin in sample.